I am working on something. I don't know if it'll be finished by Oct 31st, but I'll start posting on Oct 1st and it'll run until it's done.
Not for Halloween specifically, no. Partly because I have some other stuff already planned, competitions, etc for the occasion. The other part because my niche already fits the spooky season. Lastly and I guess this is more an admission of flaw: My writing tends to take me a while and I prefer to work on a couple projects at once so as not to lose my brainwaves and trains of thought. That means a lot of my works are usually start weeks in advance from when I publish or reveal them. I won't say it's a definite no, however. If I get a festive idea that isn't too schlocky or silly, I'll definitely put it to paper (or internet, in this case).
It's not really horror, but I've been kicking around a Halloween themed story in mind for a while now and it would be great to get it posted during the month. I just need to buckle down and finish it on time.